When exposed to abuse from high temperatures, overcharging, or short-circuits, lithium-ion batteries can catch fire and explode. The flammable nature and low flashpoints of the organic carbonate-based solvents comprising the electrolyte make carbon anodes soaked in electrolyte vulnerable to self-heating at temperatures as low as 50° C. Furthermore, the most common electrolyte salt, lithium hexafluorophosphate (LiPF6), has poor stability at elevated temperatures and can autocatalyze the decomposition of the electrolyte solvent. Several high profile incidents resulting from lithium-ion battery overheating and fire, including the recalls of millions of laptop batteries as well as electric cars, illustrate that lithium-ion battery failures can present a high risk of economic loss.
In terms of engineering controls for battery safety, several strategies exist at the system (e.g., vehicle) level, module/pack level, and at the cell level that revolve around management or prevention of decomposition of the organic liquid electrolyte. These include thermal management strategies such as liquid or air cooling; battery management systems for careful voltage, current, and temperature control, and cell level controls such as cell venting and shut down separators. Electrolyte additives that serve as internal controls have also been investigated. Flame retardant additives decrease the flammability of the liquid electrolyte, but can increase the electrochemical instability or viscosity of the liquid electrolyte, leading to lower energy capacity and power characteristics, and representing a significant portion of the battery costs.
The ionic conductivity of the most common liquid electrolyte, 1 M LiPF6 in ethylene carbonate/diethylene carbonate (EC-DEC) solvent exceeds that of several alternatives. However, liquid electrolytes typically require a microporous polymer separator (e.g., CELGARD®) to prevent the anode and cathode from touching, while allowing liquid electrolyte transport through the pores. The separators can be 20% of the total battery cost and their mechanical properties are crucial to its safety. Existing polymer separators undergo structural changes and thermal shrinkage at elevated temperatures (˜100° C.), can be easily punctured by lithium dendrites (which may short circuit the cell), and can suffer from creep-induced pore closure due to mechanical stresses on the battery or from the electrode volume changes during lithiation/delithiation.
FIG. 1A depicts lithium-ion battery (LIB) 100 with a liquid electrolyte. Lithium-ion battery 100 includes anode 102 and cathode 104. Anode 102 and cathode 104 are separated by separator 106. Anode 102 includes anode collector 108 and anode material 110 in contact with the anode collector. Cathode 104 includes cathode collector 112 and cathode material 114 in contact with the cathode collector. Electrolyte 116 is in contact with anode material 110 and cathode material 114. Anode collector 108 and cathode collector 112 are electrically coupled via closed external circuit 118. Anode material 110 and cathode material 114 are materials into which, and from which, lithium ions 120 can migrate. During insertion (or intercalation) lithium ions move into the electrode (anode or cathode) material. During extraction (or deintercalation), the reverse process, lithium ions move out of the electrode (anode or cathode) material. When a LIB is discharging, lithium ions are extracted from the anode material and inserted into the cathode material. When the cell is charging, lithium ions are extracted from the cathode material and inserted into the anode material. The arrows in FIG. 1A depict movement of lithium ions through separator 106 during charging and discharging.
When electrolyte 116 is a liquid electrolyte, separator 106 is typically a polymer separator, such as CELGARD®. Inorganic separators have also been used. Lithium-ion batteries with inorganic separators (e.g., SEPARION® ceramic separators), however, still typically require flammable liquid electrolytes, since they are generally composed of oxides (e.g. Al2O3) that do not conduct Li ions.
Alternatives to liquid electrolytes generally suffer from mechanical, electrochemical, or other shortcomings that typically make them impractical. For example, solid electrolyte alternatives with high Li+ ionic conductivity and intrinsic thermal stability have been identified; however, current methods of materials synthesis are incapable of supplying solid electrolytes with the strength and flexibility required for commercial scale production.
FIG. 1B depicts lithium-ion battery 150 with anode 102, anode collector 108, cathode 104, cathode collector 112, and solid state electrolyte 152. When LIB 150 is discharging, lithium ions are extracted from the anode material and inserted into the cathode material. When the cell is charging, lithium ions are extracted from the cathode material and inserted into the anode material. The arrows in FIG. 1B depict movement of lithium ions through solid state electrolyte 152 during charging and discharging.